The present invention relates to a 90-degree phase shifter, and more specifically to a 90-degree phase shifter used for generating a pair of quadrature carriers which are used in detecting an orthogonal signal in a digital broadcasting.
In the prior art, the 90-degree phase shifter is used for example for generating a pair of quadrature carriers (a pair of carrier waves having a 90-degree phase difference therebetween) which are used in detecting an orthogonal signal in a digital broadcasting.
Referring to FIG. 1, there is shown a block diagram illustrating the structure of one typical example of the 90-degree phase shifter disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-10-075157. The shown prior art 90-degree phase shifter includes a variable time constant low pass filter (LPF) 2 and a variable time constant high pass filter (HPF) 3, both of which receive the same input signal 1, and generate output signals 4 and 5 having a 90-degree phase difference therebetween, respectively. A phase error detection means 6A and an amplitude error detection means 6B receive the output signals 4 and 5 of the variable time constant low pass filter 2 and the variable time constant high pass filter 3 to detect a phase error and an amplitude error between the output signals 4 and 5, respectively. A control means 7 receives and synthesizes a phase error signal and an amplitude error signal outputted from the variable time constant low pass filter 2 and the variable time constant high pass filter 3, respectively, and controls respective time constants of the variable time constant low pass filter 2 and the variable time constant high pass filter 3, for the purpose of reducing the phase error and the amplitude error between the output signals 4 and 5 having a 90-degree phase difference therebetween.
On the other hand, a highly precise transmission signal is now required with a recently expedited popularization of the digital broadcasting and the digital communication, and with demands for an increase in the number of channels and in the signal density and for effective utilization of radio signals. However, the prior art cannot optimally remove the amplitude error and the phase error in the transmission signal.
Accordingly, it is an object of the present invention to provide a 90-degree phase shifter which can highly precisely remove the amplitude error and the phase error in the transmission signal.
The above and other objects of the present invention are achieved in accordance with the present invention by a 90-degree phase shifter comprising:
an input terminal, a first output terminal, and a second output terminal;
a first variable gain amplifying means, a first phase adjusting means and a low pass filter means, which are cascaded between the input terminal and the first output terminal;
a second variable gain amplifying means, a second phase adjusting means and a high pass filter means, which are cascaded between the input terminal and the second output terminal;
a phase error detection means receiving a first output signal on the first output terminal and a second output signal on the second output terminal for detecting a phase error between the first output signal and the second output signal to supply a first phase adjusting signal and a second phase adjusting signal to the first phase adjusting means and the second phase adjusting means, respectively; and
an amplitude error detection means receiving the first output signal and the second output signal for detecting an amplitude error between the first output signal and the second output signal to supply a first gain control signal and a second gain control signal to the first variable gain amplifying means and the second variable gain amplifying means, respectively,
whereby the phase error and the amplitude error between the first output signal and the second output signal are removed independently of each other.
Here, the problem of the prior art 90-degree phase shifter is examined in detail. The prior art 90-degree phase shifter has a problem that a DC bias dependency is large and variation of the time constant is large in a convergent condition. A first ground of causing this problem is considered that in the prior art 90-degree phase shifter, two error components, namely, an amplitude error xcex94V and a phase error xcex94xcfx86 are synthesized to generate a pair of control signals supplied to the pair of variable time constant filters, respectively. In other words, one variable time constant filter is supplied with only one control signal for the purpose of attempting to minimize both the amplitude error and the phase error. However, it is in some cases that it is not possible to obtain an optimum control signal capable of removing both the amplitude error xcex94V and the phase error xcex94xcfx86. In addition, it is in some cases that there is no convergence value which minimizes both the amplitude error and the phase error.
Secondly, since the variable time constant filter includes an active resistive or capacitive element whose resistance or capacitance can be controlled by an applied voltage, there is no way other than to use an element having such a large degree of bias dependency that for example the resistance value greatly varies with only a periodical change of the DC voltage of the input signal.
The above mentioned problems can be overcome by the present invention, since the phase error and the amplitude error between the first output signal and the second output signal can be removed independently of each other.
In a first embodiment of the 90-degree phase shifter in accordance with the present invention, the first variable gain amplifying means, the first phase adjusting means and the low pass filter means are cascaded in the named order between the input terminal and the first output terminal, so that an input of the first variable gain amplifying means is connected to the input terminal and an output of the low pass filter means is connected to the first output terminal, and the second variable gain amplifying means, the second phase adjusting means and the high pass filter means are cascaded in the named order between the input terminal and the second output terminal, so that an input of the second variable gain amplifying means is connected to the input terminal and an output of the high pass filter means is connected to the second output terminal.
In a second embodiment of the 90-degree phase shifter in accordance with the present invention, the first variable gain amplifying means, the low pass filter means and the first phase adjusting means are cascaded in the named order between the input terminal and the first output terminal, so that an input of the first variable gain amplifying means is connected to the input terminal and an output of the first phase adjusting means is connected to the first output terminal, and the second variable gain amplifying means, the high pass filter means and the second phase adjusting means are cascaded in the named order between the input terminal and the second output terminal, so that an input of the second variable gain amplifying means is connected to the input terminal and an output of the second phase adjusting means is connected to the second output terminal.
In a third embodiment of the 90-degree phase shifter in accordance with the present invention, the low pass filter means, the first variable gain amplifying means and the first phase adjusting means are cascaded in the named order between the input terminal and the first output terminal, so that an input of the low pass filter means is connected to the input terminal and an output of the first phase adjusting means is connected to the first output terminal, and the high pass filter means, the second variable gain amplifying means and the second phase adjusting means are cascaded in the named order between the input terminal and the second output terminal, so that an input of the high pass filter means is connected to the input terminal and an output of the second phase adjusting means is connected to the second output terminal.
In a fourth embodiment of the 90-degree phase shifter in accordance with the present invention, the first phase adjusting means, the first variable gain amplifying means and the low pass filter means are cascaded in the named order between the input terminal and the first output terminal, so that an input of the first phase adjusting means is connected to the input terminal and an output of the low pass filter means is connected to the first output terminal, and the second phase adjusting means, the second variable gain amplifying means and the high pass filter means are cascaded in the named order between the input terminal and the second output terminal, so that an input of the second phase adjusting means is connected to the input terminal and an output of the high pass filter means is connected to the second output terminal.
In a fifth embodiment of the 90-degree phase shifter in accordance with the present invention, the first phase adjusting means, the low pass filter means and the first variable gain amplifying means are cascaded in the named order between the input terminal and the first output terminal, so that an input of the first phase adjusting means is connected to the input terminal and an output of the first variable gain amplifying means is connected to the first output terminal, and the second phase adjusting means, the high pass filter means and the second variable gain amplifying means are cascaded in the named order between the input terminal and the second output terminal, so that an input of the second phase adjusting means is connected to the input terminal and an output of the second variable gain amplifying means is connected to the second output terminal.
In a sixth embodiment of the 90-degree phase shifter in accordance with the present invention, the low pass filter means, the first phase adjusting means and the first variable gain amplifying means are cascaded in the named order between the input terminal and the first output terminal, so that an input of the low pass filter means is connected to the input terminal and an output of the first variable gain amplifying means is connected to the first output terminal, and the high pass filter means, the second phase adjusting means and the second variable gain amplifying means are cascaded in the named order between the input terminal and the second output terminal, so that an input of the high pass filter means is connected to the input terminal and an output of the second variable gain amplifying means is connected to the second output terminal.
Specifically, in the 90-degree phase shifter in accordance with the present invention, the first phase adjusting means can include a first constant current source means connected in series therewith, the first controllable constant current source means being controlled by the first phase adjusting signal, and the second phase adjusting means can include a second controllable constant current source means connected in series therewith, the second controllable constant current source means being controlled by the second phase adjusting signal.
More specifically, the first phase adjusting means can include a first transistor and a first controllable constant current source means connected in series between a high voltage terminal and a ground, a control terminal of the first transistor constituting an input of the first phase adjusting means, and a connection node between the first transistor and the first controllable constant current source means constituting an output of the first phase adjusting means, the first controllable constant current source means being controlled by the first phase adjusting signal. The second phase adjusting means can include a second transistor and a second controllable constant current source means connected in series between a high voltage terminal and a ground, a control terminal of the second transistor constituting an input of the second phase adjusting means, and a connection node between the second transistor and the second controllable constant current source means constituting an output of the second phase adjusting means, the second controllable constant current source means being controlled by the second phase adjusting signal.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.